The present invention relates to a fast reactor core in which 30 to 50 target assemblies, each containing 20 to 50% of minor actinide nuclides having long half-lives, are so heterogeneously dispersed and loaded that the minor actinide nuclides can efficiently be burned up with no considerable influence on the core characteristics.
The use of a fast reactor for annihilating minor actinide nuclides of long half-lives that are present in spent nuclear fuel from light-water reactors is now under investigation. These minor actinides, for instance, include neptunium (Np), americium (Am), and curium (Cm). For loading minor actinide nuclides-containing fuel in a reactor core there are two approaches, one called the homogeneous loading of fuel in which small amounts of minor actinide nuclides are homogeneously added to and loaded in all core fuel, and the other referred to as the heterogeneous loading of fuel in which a small number of special fuel assemblies (called the target fuel assemblies) with large amounts of minor actinide nuclides added thereto are loaded in the reactor core.
It has now been found that the homogeneous loading of fuel enables minor actinide nuclides to be annihilated with no serious influence on the core characteristics, if the content of the minor actinide nuclides is reduced to about 5% on the reactor average. Since, of the minor actinide nuclides, americium and curium generate large amounts of neutrons, gamma rays and heat, however, it is required to use thickly shielded cells for fuel fabrication.
On the contrary, the heterogeneous loading of fuel enables the number of minor actinide nuclides-containing fuel elements to be reduced, and so has some merits in view of fuel fabrication and management. When this heterogeneous loading of fuel is used to load in a reactor core target fuel assemblies with high content of minor actinide nuclides (i.e., with minor actinide nuclides contained in them in amounts of 20 to 50% on the assembly average), it has now been found that the nuclear and thermal properties are improved by dispersed fuel loading rather than by concentrated fuel loading.
However, it is expected that the physical properties of minor actinide nuclides would make the melting point and thermal conductivity of fuel containing large amounts of minor actinide nuclides lower than those of ordinary mixed-oxide fuel (MOX). When target fuel assemblies containing minor actinide nuclides are merely loaded by the heterogeneous loading technique, it is expected that the fuel permissible linear power would be exceeded. To avoid this, it is required to lower the operating power and, hence, it is substantially impossible to achieve sufficient annihilation of minor actinide nuclides within the target fuel assembly.
It is thus an object of the present invention to provide a solution to the technical problems as mentioned above by the provision of a fast reactor core in which a reduced number of target assemblies containing large amounts of minor actinide nuclides having long half-lives can be heterogeneously dispersed and loaded to efficiently burn up the large amounts of minor actinide nuclides with no significant influence on the core characteristics.